[
  "AOB is a straight line (180°). Since ∠AOC = 90°, then ∠COB = 90° too.",
  "Find ∠COE = ∠AOC − ∠AOE = 90° − 71° = 19°.",
  "∠BOD is vertically opposite to ∠COE, so ∠BOD = 19°."
]